I've created a version with images.
First of all, a tip: Surround the battle arena with walls so that Boom-Boom can't escape.
Place the starting positions of Player 1 and Player 2 like you already did in your image.
In the Layers menu (View > Layers), make a new layer. I called it "Door" here.
Select this layer by clicking on it, and place the door and the warp. You know you're doing this right when the name of the layer, in this case "Door", appears next to the cursor.
In the Events menu (View > Events), add a new event. I called it "BossDefeat" here.
Select the Level - Start event, select the Door layer, and add it under "Hide Layers".
Select the BossDefeat event, select the Door layer, and add it under "Show Layers".
Now, when you want to place Boom-Boom (the boss), under "Events", click on Show, and then, in the window called "NPC Event Triggers", under "Death", click on the arrow, and select BossDefeat there.
And that's it. If you kill Boom-Boom, then the door and the warp will appear.
I hope I could help.
